Dielectric isolation fabrication for laser trimming

ABSTRACT

Single crystal dielectrically isolated islands are formed providing an opening in the bottom dielectric isolation of selected islands before the application of the polycrystalline support. Thin film resistor material is formed and delineated on an insulative layer over the single crystal island juxtaposed with the opening of the bottom dielectric isolation. The thin film resistive layer is trimmed using a laser.

BACKGROUND OF THE INVENTION

The present invention relates generally to laser trimming and morespecifically to laser trimming above a dielectrically isolated singlecrystalline region.

Laser trimming of thin-film resistors is used extensively to produceimproved accuracy in analog integrated circuit technology. Inintegrating laser trimming into dielectrically isolated circuittechnology a difficulty peculiar to dielectric isolation has beenidentified. Trimming is generally accomplished by use of an infraredlaser for improved control. Silicon is nearly transparent at thewavelengths used; this results in laser energy penetrating to the bottomof the dielectrically isolated island, reflecting back and transferringsome of the reflected energy back to the resistor. The result is poorcontrol due to interference effects. These effects are variable due tochanges in dielectrically isolated island depth and proper control overtrim energy hence becomes very difficult.

An existing technique addresses the problem by simply placing thethin-film resistor over the polycrystalline silicon used to support thedielectrically isolated regions. The polysilicon is much thicker thanthe single-crystal islands (typically 10 mils vs. 1 mil) and energyscattering off the grain boundaries soon dissipates the laser beam. Theresulting lack of reflection and interference produces enhancedcontrollability. The resistor is deposited over a polycrystallinesurface which is not perfectly flat but in fact possesses considerablerelief at the grain boundaries. In addition, large polycrystalline areastend to "dish out" which complicates laser focussing and reducesphotoresist definition, resulting in poor control over resistorgeometries. There is also concern over the validity of thin film overpolycrystalline systems for high-reliability applications.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of fabricatinglaser trimmed thin film resistors over single crystal islands.

Another object of the present invention is to provide a method of lasertrimming thin film resistors on dielectrically isolated integratedcircuits.

These and other objects of the invention are attained by fabricatingdielectrically isolated single crystal islands over which are formed thethin film resistors with an opening in the bottom of the dielectricisolation. The thin film resistor is formed over an insulative surfaceon the single crystal region and is subsequently laser trimmed. Theopening in the bottom of the dielectric isolation of the single crystalislands including a polycrystalline support structure may be formed byputting a oxide inhibiting mask over the starting substrate followed byformation of an oxide mask to define the isolation moats, formation ofmoats using the oxide mask, forming the dielectric isolation layer overthe bottom of the substrate and the face of the moats, stripping of theoxide inhibiting mask, applying the polycrystalline support structureand removing the top surface of the original substrate to form thedielectrically isolated regions. Alternatively, the opening in thebottom surface of the dielectric isolation layer may be formed bycompletely oxidizing the bottom of the substrate and the faces of themoat followed by a mask and removal step of the dielectric isolation atthe bottom of selected regions. A third alternative is to form the moatmask and the moats followed by applying an oxide inhibiting layer over aregion of the bottom surface of the substrate followed by oxidization toform the dielectric isolation except in the regions covered by the oxideinhibiting mask. This is followed by applying the polycrystallinesupport structure. After forming the dielectrically isolated islands,the top surface of the substrate is covered with an insulative layer andthe thin film resistive material is applied and delineated thereon toform a resistor juxtaposed to the opening in the bottom dielectricisolation. This is followed by laser trimming using a laser of suitablewavelength and power level.

Other objects, advantages and novel features of the present inventionwill become evident from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 7 are cross-sectional views illustrating the differentstages of fabricating a laser trimmed thin film resistor ondielectrically isolated single crystal islands according to theprinciples of the present invention.

FIGS. 8 through 11 are cross-sectional views illustrating a modificationof the process of FIGS. 1 through 7.

FIG. 12 is a cross-sectional view illustrating a further modification ofthe process of FIGS. 8 through 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of fabricating laser trimmed thin film resistors ondielectrically isolated single crystal islands begins as illustrated inFIG. 1 by epitaxially growing a single crystal layer 22 on a singlecrystalline substrate 20. Substrate 20 has a substantially higherimpurity concentration and lower resistance than the epitaxial layer 22.An oxide inhibiting mask layer 24 for example, silicon nitride, isapplied to the exposed surface 25 of the epitaxial layer 22 andpatterned to cover a region of the single crystal epitaxial layer 22.The exposed surface 25 of the epitaxially layer 22 is then subjected toan oxidizing atmosphere which forms an oxide masking layer 26 acrosssurface 25 except that region covered by the oxide inhibiting layer 24.The structure at this stage is illustrated in FIG. 2.

The mask layer 26 is delineated by standard photolithographic processesto form openings 27 therein. The epitaxial layer 22 and the substrate 20are then etched to form the V-shaped moats 28 as illustrated in FIG. 3.The oxide mask layer 26 is then removed and the exposed surface of themoats as well as the epitaxial layer 22 are exposed to an oxidizingatmosphere such that the dielectric isolation region 30 is formedthereon except on the regions covered by the oxide inhibiting mask 24.The resulting structure is illustrated in FIG. 4.

The oxide inhibiting layer 24 is then removed and a polycrystallinematerial is deposited over the dielectric isolation layer 30 and intothe opening 32 left by the removal of the oxide inhibiting mask 24. Thepolycrystalline support structure 34 is illustrated in FIG. 5. Thesubstrate 20 is removed by well-known methods which may include grindingor wet etching in combination with grinding down to the bottom of themoat regions 28 to expose the tip of the dielectric isolation layer 30to define a plurality of dielectrically isolated single crystallineregions of the original epitaxial layer 22 as illustrated in FIG. 6.

An insulation layer 36 for example, silicon dioxide, is formed on thetop surface 37 of the single crystal regions. A thin film resistormaterial 38 is then applied on the insulative layer 36 and delineated toform substantially the geometry of the thin film resistors juxtaposed tothe opening 32 in the dielectric isolation at the bottom of the singlecrystalline region 22. The final geometry and characteristic of the thinfilm resistor is defined by laser trimming the layer 38. Preferably, thelaser is an infrared laser. Since the silicon is nearly transparent atinfrared wavelengths, the laser energy penetrates through the singlecrystalline region 22 and into the polycrystalline support region 34through the opening 32 on the dielectric isolation 30. Little if any ofthe laser radiation is reflected back affecting the thin film resistor38. Since the thin film resistor 38 is formed over the single crystalregion 22, the energy scattering off the grain boundaries dissipates thelaser beam thereby minimizing reflection and interference. Similarly,the well-defined and flat surface of the single crystalline region donot result in dishing out or other problems with large polycrystallinesilicon areas.

An alternative method for forming the opening in the dielectricisolation 30 at the bottom of the single crystal region 22 isillustrated in FIGS. 8 through 11. The oxide moat mask 26 is formed onthe entire surface 25 of the epitaxial region 22 and moat openings 27are formed therein as illustrated in FIG. 8. The moat regions 28 areformed in the epitaxial layer 22 and the substrate 20 using the moatmask 26 as illustrated in FIG. 9. The mask 26 is removed and the waferis then subjected to an oxidizing atmosphere to form the dielectricisolation region 30 over the walls of the moats 28 and the bottomsurface 25 of the epitaxial region 22 as illustrated in FIG. 10. A masklayer 40 is then formed on the dielectric isolation 30 and an opening 42is provided therein. The portion of the dielectric isolation 30 exposedby the opening 42 is then removed to form the dielectric isolation 30with an opening therein exposing the bottom of the single crystal region22. The process is then completed following the steps of FIGS. 5, 6 and7.

A second alternative method of fabrication is to perform the moatmasking and etching following the steps of FIGS. 8 and 9 including thestripping of the masking layer 26. Before the formation of thedielectric isolation region 30, an oxide inhibiting layer 24 is appliedand delineated to cover a region of the bottom surface 25 of theepitaxial region 22. The resulting structure is illustrated in FIG. 12.This is followed by the oxidation to form the dielectric isolationregion 30 following the process steps of FIGS. 4 through 7.

It should be noted that the method of fabrication of FIGS. 1 through 4are preferred since no masking steps are performed after the formationof the moats 28. In the modification of the embodiments illustrated inFIGS. 11 and 12, a masking layer 40 in FIG. 11 and the oxide inhibitinglayer 24 are formed not only on the bottom surface 25 of the epitaxiallayer but is also formed in the moat regions 28. These mask regions mustbe removed and consequently the moat regions are subjected to possiblysome additional etching which may be undesirable since it may effect thespacing and accuracy of the dielectric isolation.

From the preceding description of the preferred embodiments, it isevident that the objects of the invention are attained and although theinvention has been described and illustrated in detail, it is to beclearly understood that the same is by way of illustration and exampleonly and is not to be taken by way of limitation. The spirit and scopeof this invention are to be limited only by the terms of the appendedclaims.

What is claimed is:
 1. A method of fabricating a thin film resistorcomprising:forming a plurality of single crystal islands dielectricallyisolated from a polycrystalline support, one or more of which include anopening in the bottom of said dielectric isolation to optically expose aportion of said polycrystalline support from the top surface of saidsingle crystal island; forming an isolation layer on said top surface ofsaid single crystal island; forming a layer of conductive material onsaid surface isolation layer; delineating said conductive material toform a thin film resistor juxtaposed with said opening in said bottom ofsaid dielectric isolation; and subjecting said thin film resistor tolaser radiation to further remove portions of said conductive layer. 2.The method according to claim 1, wherein forming said single crystalislands include:forming an oxide inhibiting layer on a region of thebottom surface of a single crystal semiconductor substrate; oxidizingthe remainder of said bottom surface of said single crystalsemiconductor substrate to form an oxide mask; forming moats in thebottom surface of said single crystal semiconductor substrate using saidoxide mask; removing said oxide mask; oxidizing said bottom surface andthe walls of said moats to form a dielectric isolation barrier except atthe region covered by said oxide inhibiting layer; removing said oxideinhibiting layer; forming a polycrystalline support on said dielectricisolation barrier; and removing the top surface of said single crystalsemiconductor substrate to a plane which forms dielectrically isolatedsingle crystal islands.
 3. The method according to claim 1, whereinforming said single crystal islands includes:forming a mask on thebottom surface of a single crystal semiconductor substrate exposing moatregions; forming moats in said bottom surface using said mask; removingsaid mask layer; oxidizing said bottom surface and the walls of saidmoats to form a dielectric isolation barrier; removing a portion of saidisolation barrier from the bottom surface of said dielectric isolationbarrier; forming a polycrystalline support on said dielectric isolationbarrier; and removing the top surface of said single crystalsemiconductor substrate to a plane which forms dielectrically isolatedsingle crystal islands.
 4. The method according to claim 1, whereinforming said single crystal islands includes:forming a mask on thebottom surface of a single crystalline semiconductor substrate exposingmoat regions; forming moats in said bottom surface using said mask;removing said mask layer; forming an oxide inhibiting layer on a regionof the bottom surface of said single crystal semiconductor substrate;oxidizing said walls of said moats and said bottom surface except forthe region covered by said oxide inhibiting layer to form a dielectricisolation barrier; removing said oxide inhibiting layer; forming apolycrystalline support on said dielectric isolation barrier; andremoving the top surface of said single crystal semiconductor substrateto a plane which forms dielectrically isolated single crystal islands.5. The method according to claim 1, wherein said laser radiation isinfrared laser radiation.